Thermostat has no power. Went up in the attic and looked at the furnace unit. Has an ST9141 unit. I actually have two separate units. On the working one there is a steady green light on the 9141. On the non working one there is no light whatsoever. Totally dark. I went to the outside AC unit and the compressor and fan fired up just fine when I pushed the center of the contactor block in so it is not a problem with the AC unit. If I stood there and held the contactor in it would continue to work. Somehow I am not getting any power to the thermostat so it cannot turn the unit on. Any ideas? The transformer is a 24 Volt one and I did not see a fuse anywhere.

THANKS!

Check for 24 volts coming off the board inside the furnace panel. This should help. https://www.askmehelpdesk.com/heating-air-conditioning/heating-cooling-maintenance-repair-troubleshooting-faq-northerheat-58313.html What type of stat do you have, is there also a battery backup?

The battery had run out and the power indicator on the thermostat was dark. I used a voltmeter on the transformer and there is 120V going in and zero coming out. I assume that means that the transformer is bad. I can get my leads on the terminals with it hooked up so it was not too hard to check. Do you also think it is the transformer? Is there something else I should check? I guess my main question is, did something make the transformer go out, or is this a normal problem without any other causes?

Thanks for the response!

Unusual for both to go out. You tested between the R or Rh and ground and got zero, right? Replace the battery first, it likely powers your display.

I forgot to mention that before I figured the transformer was bad I went out and bought a new thermostat and installed it. It made absolutely no difference. It lit up with the batteries installed in it but did not work. When I pulled the batteries out, it didn't do anything.

I'm not an expert on this stuff but I had a problem with the thermostat not working years ago. Long story short- it turned out that a mouse had chewed the wire down in the wall at some time. It was real simple once I realized what was going on.

If you are testing for 24 volts off the mother board then we can skip thinking mice. Tell us where and which wires you tested. I think you are in the market for a new transformer. Amazon.com: Honeywell #AT72D-1683 120V/24V Transformer: Home Improvement (http://www.amazon.com/Honeywell-AT72D-1683-120V-24V-Transformer/dp/B000692A58)

Make sure you have voltage going to the air handler. Check the breakers.. If not sure if its triped turn the breaker off and then back on. If that does not help its time to get out the volt meter to get some readings.

OP has said he has 120 feeding the transformer.